chore: prepare the 1.2.0 release

Version 1.2.0 (manifest "1.2"): the release adds features without
breaking changes, and 1.1.0 already exists in the changelog. The
lockfile's root version, still 1.0.0, moves with it.

Turn Unreleased into the 1.2.0 section, fold in the unheaded bullets
beneath it, add the template-limit entry, note that 1.1.0 never reached
addons.mozilla.org, and fix the stray ")" in the 1.0.0 heading. The
manifest description now covers the menu, toolbar button and shortcut.

Record in the design spec that ChatGPT auto-submits ?q= prompts again as
of 2026-10-18, and what that means for Appendix B's selection-smuggling
argument.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "search-with-chatgpt-powered-by-openai-extension",
-  "version": "1.1.0",
+  "version": "1.2.0",
   "private": true,
   "type": "module",
   "description": "Development tooling for the Search with ChatGPT Firefox extension.",